This invention relates generally to gas anesthesia machines, and, more particularly, to alarm and control systems for use in anesthesia machines to ensure that a patient receives an adequate supply of oxygen while undergoing anesthesia, and that the patient is breathing in an uninterrupted breathing cycle.
A gas anesthesia machine includes an oxygen supply, a supply of anesthesia gas, such as a nitrous oxide (N.sub.2 O) flow control valves and flow meters for the anesthesia gas and the oxygen, and a common outlet by means of which a mixture of the gases is passed to a patient breathing machine. The patient breathing machine, which forms no part of the present invention, is typically a closed-circuit system including a carbon dioxide absorber and at least two check valves, to ensure that the patient inhales gas from the common outlet and exhales through the carbon dioxide absorber. The patient breathing machine may also include a ventilator to pump breathing gas into the patient's lungs, and a gas evacuation system for removal of excess gas from the patient breathing machine.
In simple terms, a gas anesthesia machine provides a mixture of anesthesia gas and oxygen in proportions selected by an operator, who is usually a doctor. The doctor manipulates the flow control valves and monitors the respective flows of the gases through the flow meters. The principal problem with which the present invention is concerned is that the flow of oxygen may inadvertently fall below a danger level, and that the patient may suffer serious physiological damage, or even death, as a result of the oxygen deficiency. The doctor may, for example, become more concerned with increasing the flow of nitrous oxide to further anesthetise the patient, and may not notice that the percentage of oxygen has fallen below the danger level. Various techniques have been devised to provide an alarm indication when the percentage of oxygen falls below a selected level. However, such systems have suffered significant disadvantages, and have provided no means for ensuring that a minimum percentage of oxygen is delivered to the patient.
For example, a low-oxygen alarm device of the prior art is disclosed in U.S. Pat. No. 4,191,952 issued in the name of Schreiber et al. In the Schreiber device, oxygen pressure is applied to one side of a piston and nitrous oxide pressure is applied to the other. By means of springs within the device, and a proper choice of flow restrictors for the two gases, the piston is balanced in a neutral position if the desired proportion of oxygen flow is obtained. If the oxygen flow falls below a selected level, the piston is translated sufficiently in one direction to actuate an alarm switch. One difficulty in devices of this kind is that the spring pressures and flow restrictors must be selected and maintained accurately in order for the device to provide a reliable alarm at the desired flow rate. More significantly, the device does not work well at relatively low flow rates, when the oxygen flow is below one liter per minute.
Flow restrictors in the oxygen and nitrous oxide lines, between the control valves and the flow meters, provide pressures high enough to operate the piston of the device. For the most part, the flow rate through these restrictors increases linearly with the pressure on the supply side of the restrictors. However, nitrous oxide has a distinctly non-linear pressure-flow characteristic at verv low flow rates, and the pressure is significantly lower than would be expected if the linear relationship applied. Consequently, at low flows the nitrous oxide restrictor indicates a nitrous oxide flow rate that is lower than the actual flow rate, and the alarm is actuated when the percentage of oxygen is actually much lower than the desired safety level.
A commonly selected minimum safety level is twenty-six percent of oxygen, but for low flows the alarm would be actuated at percentages of oxygen as low as ten percent unless some adjustment or compensation were made to the detector system. In the Schreiber device, the piston is biased in such a manner that, at low flows, the alarm actuation level is higher than twenty-six percent, and sometimes as high as forty percent. While this approach does provide an alarm indication at a conservatively high safety level in low flow conditions, this is not always consistent with what is desired by the doctor. In fact, there may be a tendency for the doctor to switch off the alarm in such conditions, with the consequent risk that the alarm might remain off when a true low-oxygen condition is present at some later time. Ideally, then, the low-oxygen alarm should be actuated consistently at the same selected percentage of oxygen for all flow rates, even down to an extremely low flow rate.
Another significant drawback of the prior art is that the doctor has no way of guaranteeing that a desired proportion of oxygen will be delivered to the patient. In some situations, the alarm might be ignored, or might be malfunctioning for some reason, and the oxygen level could fall substantially below the safety level, unnoticed by the doctor. Thus, there is a real need for a system for controlling the oxygen flow rate to maintain it at or above the selected safety level.
Another important aspect of alarm systems for use in conjunction with gas anesthesia machines is that there should be some means for monitoring the pressure of the gas delivered to the patient. It is common practice in anesthesia machines to employ a pressure sensor located in or near the patient airway or breathing tube, to monitor the variations in airway pressure during successive breathing cycles. When the airway pressure is above a selected threshold value, a pressure-actuated electrical switch is closed, and when the pressure falls below the threshold value the switch opens again. Prior art systems have monitored this type of switch to ensure that successive closures of switches are spaced apart by no more than a preselected time interval. Failure to detect a closure within the time interval results in the actuation of an alarm. This technique reliably detects any break or substantial leak in the breathing system delivering gas to the patient. It does not, however, detect any blockage in the system, since a blockage will keep the switch in a closed condition that will not actuate the alarm.
As will be appreciated from the foregoing, gas anesthesia machines of the prior art have a number of disadvantages in relation to the detection, alarming, and control of undesirable oxygen and breathing gas conditions. The present invention is directed to a solution to these problems.